Reasons why it waggles and shifts about (7)

I believe the answer is:

motives

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'reasons why' is the definition.
(I've seen this in another clue)

I don't understand how the rest of the clue works.

(Other definitions for motives that I've seen before include "Reasons for action" , "They're why" , "inducements" .)

I've seen this clue in the Evening Standard.

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